Let’s talk about Unknown Computer Support in ConfigMgr Current Branch. I’m personally not a big fan of this feature, but some customer have a requirement to enable this option and that’s OK .

In one case we had to enable this because the customer missed to get a hardware list from his hardware vendor and was not able to do a bulk import of the devices to ConfigMgr. So we decided to build a service which is using PowerShell in a WIM to create a computer asset in the CMDB. The other reason to enable “Unknown Computer” support can be, the issue with DELL SMBIOS GUIDs.
If you boot some DELL devices (also new models like Latitude 7280), you receive a different SMBIOS GUID on the Screen of the Device compared to the one you get with WMI or in the SMSPXE.log of the Distribution Point. Here an example:

DELL Screen:

SMSPXE.log

This can be a reasons to enable Unknown Computer support. But I’m still not a fan of. Why? Normally when you PXE boot a Unknown Computer it will create a new entry in the Console called “Unknown”. To find all the current Unknown devices select the Devices and filter them by “Unknown”:

But not all devices are listet – for some reason and I couldn’t find out why. Reply to me if you have some information’s about.

If you now would like to boot the device again as Unknown, you won’t be able and the SMSPXE.log will show you a “Rejected” message for the specific SMBIOS GUID.

No advertisement found, No boot action, Rejected, Not serviced

You can do any queries to find the MAC or the SMBIOS GUID of the device in the Console (GUI), but you won’t find any entries.
To get that sorted, start the SQL Management Studio and navigate to your CM_SiteCode Database. There select “Tables” and scroll down to the table called dbo.LastPXEAdvertisement. You can right click the table and show the first 1000 entries. This will list you a few entries and hopefully your SMBIOS will be listet there.

Run the following query to get your SMBIOS GUID (for your own use, change the bold entries, DB name and GUID)

This is how you will be able to boot the device again as Unknown Computer. Each PXE flag also get an Advertisement ID from your deployed Task Sequence. You can find this entry in the same table called LastPXEAdvertisementID.